Continuous process for producing anionic clay

ABSTRACT

A continuous process for the synthesis of anionic clays with carbonate and/or hydroxide anions as the charge-balancing interlayer species is disclosed. The process involves reacting a slurry comprising aluminum trihydrate and/or its calcined form, with a magnesium source. There is no necessity to wash or filter the product. It can be spray dried directly to form microspheres or it can be extruded to form shaped bodies. The product can be combined with other ingredients in the manufacture of catalysts, absorbents, pharmaceuticals, cosmetics, detergents, and other commodity products.

This is a Continuation-in-Part of application Ser No. 09/021,839 filed Feb. 11, 1998, now abandoned. The entire disclosure of the prior application is hereby incorporated by reference herein in its entirety.

BACKGROUND OF THE INVENTION

This invention involves the continuous preparation of anionic clays and the preparation of Mg—Al solid solutions by heat-treatment of the anionic clay. Anionic clays have a crystal structure which consists of positively charged layers built up of specific combinations of metal hydroxides between which there are anions and water molecules. Hydrotalcite is an example of a naturally occurring anionic clay, in which carbonate is the predominant anion present. Meixnerite is an anionic clay wherein hydroxyl is the predominant anion present.

In hydrotalcite-like anionic clays the brucite-like main layers are built up of octahedra alternating with interlayers in which water molecules and anions, more particularly carbonate ions, are distributed. The interlayers may contain anions such as NO₃ ⁻, OH, Cl⁻, Br⁻, I⁻, SO₄ ²⁻, SiO₃ ²⁻, CrO₄ ²⁻, BO₃ ²⁻, MnO₄ ⁻, HGaO₃ ²⁻, HVO₄ ²⁻, ClO₄ ⁻, BO₃ ²⁻, pillaring anions such as V₁₀O₂₆ ⁻⁶ and Mo₇O₂₄ ⁶⁻, monocarboxylates such as acetate, dicarboxylates such as oxalate, alkyl sulphonates such as laurylsulphonate.

It should be noted that a variety of terms are used to describe the material which is referred to herein as an anionic clay. Hydrotalcite-like and layered double hydroxide are interchangeably used by those skilled in the art. Herein the materials are referred to as anionic clays, comprising within that term hydrotalcite-like and layered double hydroxide materials.

The preparation of anionic clays has been described in many prior art publications.

Recently, two major reviews of anionic clay chemistry were published in which the synthesis methods available for anionic clay synthesis have been summarized, F. Cavani et al “Hydrotalcite-type anionic clays: Preparation, Properties and Applications,” Catalysis Today”, 11 (1991) Elsevier Science Publishers B. V. Amsterdam.

J P Besse and others “Anionic clays:trends in pillary chemistry, its synthesis and microporous solids”(1992), 2, 108, editors: M. I. Occelli, H. E. Robson, Van Nostrand Reinhold, N.Y.

In these reviews the authors state that a characteristic of anionic clays is that mild calcination at 500° C. results in the formation of a disordered MgO-like product. The disordered MgO-like product is distinguishable from spinel (which results upon severe calcination) and from anionic clays. Herein, the disordered MgO-like materials are referred to as Mg—Al solid solutions. Furthermore, these Mg—Al solid solutions contain a well-known memory effect whereby the exposure to water of such calcined materials results in the reformation of the anionic clay structure.

For work on anionic clays, reference is given to the following articles:

Helv. Chim. Acta, 25, 106-137 and 555-569 (1942)

J. Am. Ceram. Soc., 42, no. 3, 121 (1959)

Chemistry Letters (Japan), 843 (1973)

Clays and Clay Minerals, 23, 369 (1975)

Clays and Clay Minerals, 28, 50 (1980)

Clays and Clay Minerals, 34, 507 (1996)

Materials Chemistry and Physics, 14, 569 (1986).

In addition there is an extensive amount of patent literature on the use of anionic clays and processes for their preparation.

European Patent Application 0 536 879 describes a method for introducing pH-dependent anions into the clay. The clay is prepared by the addition of a solution of Al(NO₃)₃ and Mg(NO₃)₂ to a basic solution containing borate anions. The product is then filtered, washed repeatedly with water, and dried overnight. Additionally mixtures of Zn/Mg are used.

In U.S. Pat. No. 3,796,792 by Miyata entitled “Composite Metal Hydroxides” a range of materials is prepared into which an extensive range of cations is incorporated, including Sc, La, Th, In. etc. In the examples given solutions of the divalent and trivalent cations are prepared and mixed with base to cause co-precipitation. The resulting products are filtered, washed with water, and dried at 80° C. Example 1 refers to Mg and Al and Example 2 to Mg and Bi. Other examples are given, and in each case soluble salts are used to make solutions prior to precipitation of the anionic clay at high pH.

In U.S. Pat. No. 3,879,523 by Miyata entitled “Composite Metal Hydroxides” also a large number of preparation examples is outlined. The underlying chemistry, however, is again based on the co-precipitation of soluble salts followed by washing and drying. It is Important to emphasize that washing is a necessary part of such preparations, because to create a basic environment for co-precipitation of the metal ions a basic solution is needed and this is provided by NaOH/Na₂CO₃ solutions. Residual sodium, for example, can have a significant deleterious effect on the subsequent performance of the product as a catalyst or oxide support.

In U.S. Pat. No. 3,879,525 (Miyata) very similar procedures are again described.

In U.S. Pat. No. 4,351,814 to Miyata et al. a method for making fibrous hydrotalcite is described. Such materials differ in structure from the normal plate-like morphology. The synthesis again involves soluble salts. For example, an aqueous solution of a mixture of MgCl₂ and CaCl₂ is prepared and suitably aged. From this a needle-like product Mg₂(OH)₃Cl.4H₂O precipitates. A separate solution of sodium aluminate is then reacted in an autoclave with the solid Mg₂(OH)₃Cl.4H₂O and the product is again filtered, washed with water, and dried.

In U.S. Pat. No. 4,458,026 to Reichle, in which heat-treated anionic clays are described as catalysts for aldol condensation reactions, again use is made of magnesium and aluminum nitrate salt solutions. Such solutions being added to a second solution of NAOH and Na₂CO₃. After precipitation the slurry is filtered and washed twice with distilled water before drying at 125° C.

In U.S. Pat. No. 4,656,156 to Misra the preparation of a novel absorbent based on mixing activated alumina and hydrotalcite is described. The hydrotalcite is made by reacting activated MgO (prepared by activating a magnesium compound such as magnesium carbonate or magnesium hydroxide) with aqueous solutions containing aluminate, carbonate and hydroxyl ions. As an example the solution is made from NAOH, Na₂CO₃ and Al₂O₃. In particular, the synthesis involves the use of industrial Bayer liquor as the source of Al. The resulting products are washed and filtered before drying at 105° C.

In U.S. Pat. No. 4,904,457 to Misra a method is described for producing hydrotalcites in high yield by reacting activated magnesia with an aqueous solution containing aluminate, carbonate, and hydroxyl ions.

The methodology is repeated in U.S. Pat. No. 4,656,156.

In U.S. Pat. No. 5,507,980 to Kelkar et at al a process is described for making novel catalysts, catalyst supports, and absorbers comprising synthetic hydrotalcite-like binders. The synthesis of the typical sheet hydrotalcite involves reacting pseudo-boehmite to which acetic acid has been added to peptize the pseudo-boehmite. This is then mixed with magnesia. More importantly, the patent summary states clearly that the invention uses mono carboxylic organic acids such as formic, propionic and isobutyric acid. In this patent the conventional approaches to preparing hydrotalcite are presented.

In U.S. Pat. No. 5,439,861 a process is disclosed for preparing a catalysts for synthesis gas production based on hydrotalcite. The method of preparation is again based, on the co-precipitation of soluble salts by mixing with base, for example, by the addition of a solution of RhCl₃, Mg(NO₃)₂ and Al(NO₃)₃ to a solution of Na₂CO₃ and NAOH.

Also in U.S. Pat. No. 5,399,537 to Bhattacharyya in the preparation of nickel-containing catalysts based on hydrotalcite use is made of the co-precipitation of soluble magnesium and aluminum salts.

In U.S. Pat. No. 5,591,418 to Bhattacharyya a catalyst for removing sulphur oxides or nitrogen oxides from a gaseous mixture is made by calcining an anionic clay, the anionic clay having been prepared by co-precipitation of a solution of Mg(NO₃)₂, Al(NO₃)₃ and Ce(NO₃)₃. The product again is filtered and repeatedly washed with de-ionized water.

In U.S. Pat. No. 5,114,898/WO 9110505 Pinnavaia et al. describe layered double hydroxide sorbents for the removal of sulphur oxide(s) from flue gases, which layered double hydroxide is prepared by reacting a solution of Al and Mg nitrates or chlorides with a solution of NAOH and Na₂CO₃. In U.S. Pat. No. 5,079,203/WO 9118670 layered double hydroxides intercalated with polyoxo anions are described, with the parent clay being made by co-precipitation techniques.

In U.S. Pat. No. 5,578,286 in the name of Alcoa a process for the preparation of meixnerite is described. The meixnerite may be contacted with a dicarboxylate or polycarboxylate anion to form a hydrotalcite-like material. In comparative examples 1-3 hydromagnesite is contacted with aluminum trihydrate in a CO₂ atmosphere, greater than 30 atmospheres. No hydrotalcite was obtained in these examples.

In U.S. Pat. No. 5,514,316 a method for the preparation of meixnerite is described using magnesium oxide and transition alumina. For comparative purposes aluminum trihydrate was used in combination with magnesium oxide. It was indicated that this method did not work as well as with transition alumina.

U.S. Pat. Nos. 4,454,244 and 4,843,168 describe the use of pillaring anions in anionic clays.

In U.S. Pat. No. 4,946,581 to van Broekhoven co-precipitation of soluble salts such as Mg(NO₃)₃ and Al(NO₃)₃ with, and without the incorporation of rare earth salts was used for the preparation of anionic clays as catalyst components and additives. A variety of anions and di- and tri-valent cations are described.

As indicated in the description of the prior art given-above, there are many applications of anionic clays.

These include but are not restricted to: catalysts, adsorbents, drilling muds, catalyst supports and carriers, extenders and applications in the medical field. In particular van Broekhoven has described their use in SO_(x) abatement chemistry.

Because of this wide variety of large-scale commercial applications for these materials, new processes utilizing alternative inexpensive raw materials and which can be carried out in continuous mode are needed to provide a more cost-effective and environmentally compatible processes for making anionic clays. In particular, from the prior art described above one can conclude that the preparation process can be improved in the following ways: the use of cheaper sources of reactants, processes for easier handling of the reactants, so that there is no need for washing or filtration, eliminating the filtration problems associated with these fine-particled materials, the avoidance of alkali metals (which can be particularly disadvantageous for certain catalytic applications): In prior art preparations organic acids were used to peptize alumina. The use of organic acids is expensive and introduces an additional step in the synthesis process and is therefore not cost-effective. Further, in drying or calcining the anionic clay prepared by prior art processes gaseous emissions of nitrogen oxides, halogens, sulphur oxides, etc. are encountered which cause environmental pollution problems. Moreover, none of the preparation methods described in the prior art provide continuous processes for the preparation of anionic clays.

SUMMARY OF THE INVENTION

The invention includes a process for producing anionic clays using raw materials which are inexpensive and utilizing such raw materials in a simple process which is extremely suitable to be carried out in continuous mode. The process involves reacting mixtures with or without stirring in water at ambient or elevated temperature at atmospheric or elevated pressure. Such continuous processes can be operated in standard industrial equipment. More specifically, there is no need for washing or filtering, and a wide range of ratios of Mg/Al in the reaction product is possible.

In the continuous process according to the invention an aluminum source and a magnesium source, for instance magnesium oxide or brucite, are fed to a reactor and reacted in aqueous suspension to obtain an anionic clay. This invention involves the use of alumina trihydrate (such as gibbsite, bayerite or nordstrandite) or thermally treated forms thereof as aluminum source. The reaction is carried out at ambient or elevated temperature and ambient or elevated pressure and the reaction mixture results in the direct formation of an anionic clay which can be obtained by simply drying the slurry continuously retrieved from the reactor. The powder X-ray diffraction pattern (PXRD) suggests that the product is comparable to anionic clays made by other standard (batch) methods. The physical and chemical properties of the product are also comparable to those anionic clays made by the other conventional methods. The overall process of this invention is very flexible, enabling a wide variety of anionic clay compositions and anionic clay-like materials involving for example carbonate, hydroxide and other anions to be prepared in an economically and environmental-friendly manner.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a PXRD pattern of commercially available Mg—Al carbonate anionic clay.

FIG. 2 shows a PXRD pattern of a Mg—Al carbonate anionic clay prepared by coprecipitation.

FIG. 3 shows a PXRD pattern of a Mg—Al acetate anionic clay prepared by the process according to the invention, wherein gibbsite and MgO are reacted at 65° C.

DETAILED DESCRIPTION OF THE INVENTION

This invention involves the continuous preparation of anionic clays. In particular it describes a process for the preparation of an anionic clay wherein an aluminum source and a magnesium source are fed to a reactor and reacted in aqueous suspension to obtain an anionic clay, the aluminum source being aluminum trihydrate or its thermally treated forms. The magnesium source may be composed of a solution of a magnesium salt, a solid magnesium-bearing compound or a mixture of the two. Reaction between the Mg source and aluminum trihydrate or its thermally treated form results in the direct formation of an anionic clay. The reaction takes place at room temperature or higher. At temperatures higher than 100° C., the reaction is preferably carried out under autogeneous conditions. In the method according to the invention carbonate, hydroxyl. or other anions or mixtures thereof, either provided within the reaction medium for example by feeding a soluble salt to the reactor or absorbed during the synthesis from the atmosphere, are incorporated into the interlayer region as the necessary charge-balancing anion.

Anionic clays prepared by this method exhibit the well known properties and characteristics (e.g. chemical analysis, powder X-ray diffraction pattern, FTIR, thermal decomposition characteristics, surface area, pore volume, and pore size distribution) usually associated with anionic clays prepared by the customary and previously disclosed methods.

Upon being heated, anionic clays generally form Mg—Al solid solutions, and at higher temperatures, spinels. When used as a catalyst, an adsorbent (for instance a SO_(x) adsorbent for catalytic cracking reactions), or a catalyst support, the anionic clay according to the invention is usually heated during preparation and is thus in the Mg—Al solid solution form. During use in a FCC unit, the catalyst or adsorbent is converted from an anionic clay into Mg—Al solid solutions.

Therefore, the present invention is also directed to a process wherein an anionic clay prepared by the continuous process according to the invention, is heat-treated at a temperature between 300 and 1200° C. to form a Mg—Al-containing solid solution and/or spinel.

The anionic clay according to the invention has a layered structure corresponding to the general formula

[Mg_(m) ²⁻Al_(n) ³⁻(OH)_(2m−2n).](X_(n/z) ^(z−)). bH₂O

Wherein m and n have a value such that m/n=1 to 10, preferably 1 to 6 , and b has a value in the range of from 0 to 10, generally a value of 2 to 6 and often a value of about 4. X may be CO₃ ²⁻, OH⁻ or any other anion normally present in the interlayers of anionic clays. It is more preferred that m/n should have a value of 2 to 4, more particularly a value close to 3.

Since the continuous process disclosed herein does not require washing of the product or filtering, there is no filtrate waste or gaseous emissions (e.g. from acid decomposition), making the process particularly environmental-friendly and more suited to the environmental constraints which are increasingly imposed on commercial operations. The product can be spray dried directly to form microspheres or can be extruded to form shaped bodies.

Aluminium Source

The present invention includes the use of crystalline aluminum trihydrate (ATH), for example gibbsites provided by Reynolds Aluminum Company RH-20® or J M Huber Micral ® grades. Also BOC (Bauxite Ore Concentrate), bayerite and nordstrandite are suitable aluminum trihydrates. BOC is the cheapest alumina source. The alumina trihydrate is preferred to have a small particle size. In another embodiment of the invention thermally treated forms of gibbsite are used. Combinations of aluminum trihydrate and thermally treated forms of aluminum trihydrate can also be used. The calcined aluminum trihydrate is readily obtained by thermally treating aluminum trihydrate (gibbsite) at a temperature ranging from 100 to 800° C. for 15 minutes to 24 hours. In any event, the calcining temperature and time for obtaining calcined aluminum trihydrate should be sufficient to cause a measurable increase of the surface area in view of the surface area of the gibbsite as produced by the Bayer process which is generally between 30 and 50 m²/g. It should be noted that within the concept of this invention flash calcined alumina is also considered to be a thermally treated form of aluminum trihydrate, although generally it is considered a totally different alumina. Flash calcined alumina is obtained by treating aluminum trihydrate at temperatures between 800-1000° C. for very short periods of time in special industrial equipment, as is described in U.S. Pat. Nos. 4,051,072 and 3,222,129. Combinations of various thermally treated forms of aluminum trihydrate can also be used. Preferably the aluminum source is fed to the reactor in the form of a slurry. In particular we emphasize that there is no need to use a peptizable alumina source (gibbsite is not peptizable) and as a result no need to add either mineral or organic acid to vary the pH of the mixture. In the process according to our invention other aluminum sources beside aluminum trihydrate or its thermally treated forms may be fed to the reactor such as oxides and hydroxides of aluminum, (e.g. sols, flash calcined alumina, gels, pseudo-boehmite, boehmite) aluminum salts such as aluminum nitrate, aluminum chloride, aluminum chlorohydrate and sodium aluminate. Said other aluminum sources may be soluble or insoluble in water and may be added to the aluminum trihydrate and/or its thermally treated form or it may be fed to the reactor separately as a solid, a solution or as a suspension.

Magnesium Source

Mg-bearing sources which may be used include MgO, Mg(OH)₂, magnesium acetate, magnesium formate, magnesium hydroxy acetate, hydromagnesite (Mg₅(CO₃)₄(OH)₂), magnesium carbonate, magnesium bicarbonate, magnesium nitrate, magnesium chloride, dolomite and sepiolite. Both solid Mg sources and soluble Mg salts are suitable. Also combinations of Mg sources may be used. The magnesium source may be fed to the reactor as a solid, a solution, or, preferably, as a slurry. The magnesium source may also be combined with the aluminum source before it is fed to the reactor.

Conditions

Because of its simplicity, this process is particularly suitable to be carried out in a continuous mode. Thereto an aluminum source and a magnesium source are fed to a reactor and reacted in aqueous suspension to obtain an anionic clay.

Within the context of this invention a reactor is considered to be any confined zone in which the reaction between the aluminum source and magnesium source takes place. The reactor may be equipped with stirrers, baffles etc. to ensure homogeneous mixing of the reactants. The reaction can take place with or without stirring, at ambient or at elevated temperature and at atmospheric or elevated pressure. Usually, a temperature between 0 and 100° C. is used at or above atmospheric pressure. It is preferred to carry out the process at temperatures above 50° C. rather than at room temperature, because this results in anionic clays with sharper peaks in the x-ray diffraction pattern than anionic clays obtained at room temperature. The reactor may be heated by any heating source such as a furnace, microwave, infrared sources, heating jackets (either electrical or with a heating fluid), lamps, etc.

The aqueous suspension in the reactor may be obtained by either feeding slurries of the starting materials, either combined or separate, to the reactor or adding magnesium source to a slurry of alumina trihydrate and/or its calcined form or vice versa and feeding the resulting slurry to the reactor. It is possible to treat, for instance the aluminum trihydrate slurry at elevated temperature and then add either the Mg source per se, or add the Mg source in a slurry or solution either to the reactor or the aluminum source slurry. Given particular facilities which might be available, the continuous process can be conducted hydrothermally. This is particularly advantageous, because it this is faster and a higher conversion is obtained. There is no need to wash or filter the product, as unwanted ions (e.g. sodium, ammonium, chloride, sulphate) which are frequently encountered when using other preparation methods, are absent in the product.

In a further embodiment of the invention, the process is conducted in a multi-step process, e.g. a slurry of ATH and Mg source is treated thermally in a first reactor at a mild temperature, followed by a hydrothermal treatment in a second reactor. If desired a preformed anionic clay may be added to the reactor. The preformed clay may be recycled anionic clay from the reaction mixture or anionic clay made separately by the process according to the invention or any other process.

If desired, organic or inorganic acids and bases, for example for control of the pH, may be fed to the reactor or added to either the magnesium source or the aluminum source before they are fed to the reactor. An example of a preferred pH modifier is an ammonium base, because upon drying no deleterious cations remain in the anionic clay.

If desired, the anionic clay prepared by the continuous process according to the invention may be subjected to ion exchange. Upon ion exchange the interlayer charge-balancing anions are replaced with other anions. The other anions are the ones commonly present in anionic clays and include pillaring anions such as V₁₀O₂₈ ⁻⁶, Mo₇O₂₄ ⁶⁻, PW₁₂O₄₀ ³⁻, B(OH)₄ ⁻, B₄O₅(OH)₄ ²⁻, HBO₄ ²⁻, HGaO₃ ²⁻, CrO₄ ²⁻. Examples of suitable pillaring anions are given in U.S. Pat. No. 4,774,212 which is included by reference for this purpose. The ion exchange can be conducted before or after drying the anionic clay formed in the slurry.

The continuous process of the invention provides wide flexibility in preparing products with a wide range of Mg:Al ratios. The Mg:Al ratio can vary from 0.1 to 10 , preferably from 1 to 6, more preferred from 2 to 4, and especially preferred to close to 3.

For some applications it is desirable to have additives, both metals and non-metals, such as rare earth metals, Si, P, B, group VI, group VIII, alkaline earth (for instance Ca and Ba) and/or transition metals (for example Mn, Fe, Ti, Zr, Cu, Ni, Zn, Mo, Sn), present. The metals and non-metals can easily be deposited on the anionic clay or the solid solution according to the invention or they can be added either to the alumina source or magnesium source which are fed to to the reactor or fed the reactor separately. Suitable sources of metals or non-metals are oxides, halides or any other salt such as chlorides, nitrates etc. In the case of a multi-step process the metals and non-metals may be added in any of the steps. Is can be especially advantageous for controlling the distribution of the metals and non-metals in the anionic clay.

The present invention is illustrated by the following examples which are not to be considered limitative by any means.

EXAMPLES

Comparative Examples Based on the State of the Art

Comparative Example 1

A commercially available sample of a Mg—Al carbonate anionic clay was obtained from Reheis Chemical Company. Its PXRD pattern is shown for illustration in FIG. I

D(A) 7.80 3.89 2.59 I/I₀ 100 40 35

Comparative Example 2

This comparative example illustrates the co-precipitation method where Mg and Al salt solutions are added to a solution of base. (U.S. Pat. No. 3,979,523 Assignee Kyowa Chemical Industry, Japan)

A solution containing 0.04 M Of Al(NO₃)₂.9H₂O and 0.08 M Of Mg(NO₃)₂.6H₂O in 100 ml distilled water was added dropwise and with vigorous stirring to 150 ml of distilled water containing 0.05 M of Na₂CO₃ at room temperature. Mg/Al ratio of 2.0. The pH was maintained close to 10 by the addition of 3N NAOH and the resulting slurry aged overnight at room temperature. The precipitate was separated by centrifuge, washed several times with hot distilled water then dried at 65° C. overnight.

The PXRD pattern obtained from this sample is shown in FIG. 2. The results were:

D(A) 7.84 3.90 2.56 I/I₀ 100 40 20

Thermogravimetric analysis showed three weight losses: at approximately 100, 250 and 450° C. which are ascribed to loss of physisorbed water, interlayer water and loss of CO₂ and lattice dehydroxylation.

Comparative Example 3

The product obtained from Example 1 was calcined at 500° C. for 12 h. The product gave broad X-ray diffraction lines at 45 and 63 degrees two theta similar to those obtained for samples of calcined anionic clays prepared by other established methods with a Mg:Al ratio between 2 and 5.

Comparative Example 4

The product obtained from Example 2 was calcined at 500° C. for 12 h. The product gave broad X-ray diffraction lines at 45 and 63 degrees two theta similar to those obtained for samples of calcined anionic clays prepared by other established methods with a Mg:Al ratio between 2 and 5.

Comparative Example 5

0.15 g of the product from Comparative Example 3 was added to 75 ml water at room temperature and stirred for 12 h. The product was filtered, washed and dried at 80° C. The PXRD pattern indicated that the anionic clay structure had reformed with characteristic lines at 11.5, 23.5 and 35° in the PXRD.

Comparative Example 6

0.15 g of the product from Example 4 was added to water at room temperature and stirred for 12 h. The product was filtered and dried at 80° C.

The PXRD pattern indicated that the product was similar to that for Comparative Example 5 and confirmed that the anionic clay structure had reformed.

Examples of the Invention

The anionic clays may be prepared under nitrogen or carbon dioxide-free atmosphere, so that the anionic clay predominantly comprises hydroxide rather than predominantly carbonate as charge balancing anion. It is also possible to feed carbon dioxide to the reactor so that an anionic clay results with predominantly carbonate as charge balancing anion.

The following examples illustrate either the use of a Gibbsite (Micral 916®) obtained from J M Huber as a source of alumina or the use of BOC.

Example 7

MgO in suspension and BOC with 5% of the product of Example 2 in suspension are fed to a reactor and hydrothermally treated at 170° C. The resulting product was then dried in air at 65° C. overnight.

Example 8

MgO and Gibbsite in suspension are fed to a reactor and hydrothermally treated at 240° C. The product was dried at 65° C. for 18 hours.

Example 9

Gibbsite is calcined in air at 400° C. for 2 hours. The calcined Gibbsite and MgO in suspension are fed to a reactor and treated hydrothermally at 200° C. The product dried at 100° C.

Example 10

A sample of Gibbsite is calcined in air at 400° C. for 2 hours. The calcined gibbsite is then mixed with MgO in water. The pH of the mixture is adjusted to 6 by the addition of HNO₃, and fed to a reactor in which the mixture is treated hydrothermally at 200° C. The product dried at 110° C. 

We claim:
 1. A continuous process for the preparation of anionic clays, comprising feeding an aluminum source and a magnesium source to a reactor, and reacting the aluminum source and magnesium source in an aqueous suspension that does not comprise alkali metal to obtain an anionic clay, wherein the aluminum source is at least one of aluminum trihydrate and its thermally treated form.
 2. The process according to claim 1, wherein the aluminum source is a thermally treated form of aluminum trihydrate.
 3. The process according to claim 1, wherein the aluminum source is aluminum trihydrate.
 4. The process according to claim 1, wherein in addition to at least one aluminum trihydrate and its thermally treated form, other alumina sources are fed to the reactor.
 5. The process according to claim 1, wherein the aluminum source is fed to the reactor in the form of a slurry.
 6. The process according to claim 1, wherein the magnesium source is at least one of MgO, Mg(OH)₂ and MgCO₃.
 7. The process according to claim 1, wherein the magnesium source is fed to the reactor in the form of a slurry.
 8. The process according to claim 1, wherein the reaction takes place at a temperature of between 0 and 100° C. and at or above atmospheric pressure.
 9. The process according to any claim 1, wherein the reaction takes place at a temperature above 50° C. and at or above atmospheric pressure.
 10. The process according to claim 1, wherein the reaction is conducted at a temperature of above 100° C. and at above atmospheric pressure.
 11. The process according to claim 1, wherein metals or non-metals are fed to the reactor.
 12. The process according to claim 11, wherein the metals or non-metals are added to an aluminum source slurry.
 13. The process according to claim 11, wherein the metals or non-metals are added to a magnesium source slurry.
 14. The process according to claim 1, wherein the anionic clay is subjected to an ion-exchange treatment.
 15. The process according to claim 1, wherein the anionic clay is ion exchanged with pillaring anions.
 16. A process according to claim 15, wherein said pillaring anions comprise at least one member selected from the group consisting of V₁₀O₂₈ ⁶⁻ and Mo₇O₂₄ ⁶⁻.
 17. The process according to claim 10, wherein metals or non-metals are deposited on the anionic clay.
 18. A process for the preparation of a Al—Mg-containing solid solution and/or spinel, wherein an anionic clay obtained by the process of claim 1 is subjected to a heat-treatment at a temperature between 300 and 1200° C.
 19. A continuous process for the preparation of anionic clays, comprising feeding an aluminum source and a magnesium source to a reactor, and reacting the aluminum source and magnesium source in aqueous suspension to obtain an anionic clay, the aluminum source is a thermally treated form of aluminum trihydrate. 